The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Shock absorbers are typically used in conjunction with automotive suspension systems or other suspension systems to absorb unwanted vibrations that occur during movement of the suspension system. In order to absorb these unwanted vibrations, automotive shock absorbers are generally connected between the sprung (body) and the unsprung (suspension/drivetrain) masses of the vehicle.
The most common type of shock absorbers for automobiles are mono-tube and dual-tube shock absorbers. In the mono-tube shock absorber, a piston is located within a fluid chamber defined by a pressure tube and is connected to the sprung mass of the vehicle through a piston rod. The pressure tube is connected to the unsprung mass of the vehicle. The piston divides the fluid chamber of the pressure tube into an upper working chamber and a lower working chamber. The piston includes compression valving that limits the flow of hydraulic fluid from the lower working chamber to the upper working chamber during a compression stroke. The piston also includes rebound valving that limits the flow of hydraulic fluid from the upper working chamber to the lower working chamber during a rebound or extension stroke. Because the compression valving and the rebound valving have the ability to limit the flow of hydraulic fluid, the shock absorber is able to produce a damping force that counteracts oscillations/vibrations, which would otherwise be transmitted from the unsprung mass to the sprung mass.
In a dual-tube shock absorber, a fluid reservoir is defined between the pressure tube and a reservoir tube that is positioned around the pressure tube. A base valve assembly is located between the lower working chamber and the fluid reservoir to control the flow of dampening fluid between the lower working chamber and the fluid reservoir. The compression valving of the piston is moved to the base valve assembly and is replaced in the piston by a compression check valve assembly. In addition to the compression valving, the base valve assembly includes a rebound check valve assembly. The compression valving of the base valve assembly produces the damping force during a compression stroke, and the rebound valving of the piston produces the damping force during a rebound or extension stroke. Both the compression and rebound check valve assemblies permit fluid flow in one direction, but prohibit fluid flow in an opposite direction and these check valves can be designed such that they also generate a damping force.
Together, the compression and rebound valving and/or the check valve assemblies for the shock absorber have the function of controlling fluid flow between the upper and lower working chambers of the shock absorber. By controlling the fluid flow between the two working chambers, a pressure drop is built up between the two working chambers and this contributes to the damping forces of the shock absorber. The compression and rebound valving and the check valve assemblies can be used to tune the damping forces to control ride and handling as well as noise, vibration, and harshness.
Typical passive shock absorbers provide the same magnitude of damping force regardless of the frequency of the input. For a given input velocity, the damping force generated by a conventional passive shock absorber remains the same regardless of the frequency of the input. Typically, the primary ride frequency of a passenger vehicle is in the range of 1 to 2 Hertz. When a vehicle goes over a road surface with a lower frequency input, a higher amount of damping is preferred to manage the road inputs. During handling events (where directional stability is critical), a higher amount of damping is also preferred. For example, the vehicle may be subjected to body roll during handling events. The frequency of body roll in a typical passenger vehicle commonly ranges from 2 to 4 Hertz depending on the roll-stiffness and the height of the center of gravity of the vehicle. While there are active and semi-active damper systems that change the damping of the shock absorber in real-time to address different vehicle suspension inputs, a need exists for a passive shock absorber that provides frequency dependent damping without complicated and expansive active or semi-active damper control systems.